The general requirement for room temperature operation of a semiconducting material as a nuclear detector and spectrometer is relatively large band gap energy such that thermal generation of charge carriers is kept to a minimum. Conversely, the requirement for high resolution is small band gap energy such that a large number of electron-hole pairs are created for an absorbed quantum of ionizing radiation. The material under consideration should also have a relatively high average atomic number if used in gamma ray spectroscopy to increase the gamma ray interaction probability. High charge carrier mobilities and long charge carrier lifetimes are also needed to ensure efficient charge carrier extraction and minimal effects from position-dependent charge collection. Detectors fabricated from Cadmium Zinc Telluride (CZT) meet these requirements and are used for gamma and X-ray detection. However, in addition to excellent bulk properties of the CZT single crystal, the fabrication process and structure to create the detector having electrode charge contacts is very critical to high performance of the detector device.
Metal/semiconductor contact plays an important role in determining the performance of the CZT detector device. A good metal/semiconductor contact preferably has all of the following properties, especially for a segmented detector:                a. Good adhesion.        b. Capable of preventing charge injection.        c. Capable of preventing the inclusion of “oxides” beneath the metal—an intermediate oxide layer sandwiched in between the metal and the CZT will lower the barrier height and potentially cause polarization, having a negative effect on detector performance.        d. Reliable for assembly processes, including low temperature soldering.        
Heretofore these criteria have not been met in conventional CZT radiation detector technology. CdZnTe (CZT), and particularly Cd(1-x) Znx Te (where x is less than or equal to 0.5), is a wide band gap ternary II-VI compound semiconductor that, because of its unique electronic properties, is desirable for use in gamma-ray and X-ray spectrometers that operate at room temperature for nuclear radiation detection, spectroscopy and medical imaging applications. However, the performance of gamma-ray and x-ray segmented radiation detectors used in imaging applications and fabricated from CZT crystals is often limited because conventional fabrication processes do not achieve all four of the desired contact properties. Typically these devices have pixilated electrode arrays fabricated from various deposition and lithography processes with a gap between pixels, called the interpixel gap or region. Interpixel leakage currents act as a source of noise that reduces the ability of these spectrometers to resolve spectrally the unique radiological emissions from a wide variety of radioactive isotopes—i.e. the energy resolution (ER). The so-called interpixel resistance is a key limitation to performance and is typically much lower than overall device resistivity. Thus, in order to improve the spectral resolution capability of devices based on CZT crystals it is desirable to decrease interpixel leakage currents and the attendant detrimental noise effects.
It is known that for a semiconductor crystal to function effectively as a good detector material (i.e., minimizing interpixel surface leakage currents, thereby maximizing energy resolution) the crystal surfaces in the interpixel gap should have resistivity equal or higher than that of the bulk crystal. Generally, the interpixel surface quality is a function of the metal selected, deposition process and lithography process.
A small number of companies worldwide currently produce CZT detectors commercially in a variety of sizes and thicknesses. Usually one or both sides of the planar detectors are contacted with a continuous metal layer such as gold (Au) or platinum (Pt). As mentioned above, such detector substrates then need to be processed to produce a detector having a pattern of segmented contacts (e.g. pixel pads) on one surface, with the opposite surface remaining uniformly metallized. This is done so that the detector is able to produce a detector output indicating the position at which radiation impacts the detector.
It be believed that commercial pixilated or segmented CZT devices have been fabricated by the inverse lithography (or “lift off”) method, with its inherent performance limitations. Also, conventional deposition and lithography processes do not effectively prevent the inclusion of “oxides” beneath the metal during the process, which has a deleterious effect on device band gap. For the specific case of CZT and gold electrodes, it is not known how to use gold contact fabrication with direct photolithography such that the resulting contacts provide good adhesion and stable excellent detector performance at the same time. Poor adhesion of metal electrodes frequently causes very serious electrode lift-off problems leading to failure of the device and limited long-term reliability. Others have found that the surface resistivity of cadmium-based substrates is degraded when the substrate is exposed to conventional metal etchants and etching processes suitable for removing gold. As a result of this, the electrical separation of the individual contacts which results from the conventional method of forming contacts is not as good as would have been expected from the bulk properties before treatment. The inverse-lithography process can be used to reduce etching damage, but has not resulted in excellent interpixel resistivity combined with adequate gold adhesion of the contacts, due to limitations of the process. An example of the inverse-lithography process is U.S. Pat. No. 6,410,922, which requires additional passivation layers to facilitate the lift-off of the interpixel gap and the electrodes to overlap the passivation material. The poor contact adhesion provided by this method makes conventional attachment by methods such as low temperature soldering difficult.
There is a need to reduce the interpixel surface leakage current in CZT crystals fabricated with metal electrodes by increasing interpixel resistance in order to improve spectral resolution. There is also a need for an enhanced contact-forming process demonstrating excellent adhesion to the CZT surface, creating an electrode that is blocking and that does not introduce oxides between the metal and CZT.
Additionally, there is a need for contact-forming processes to deposit additional electrode layers on the fabricated CZT with metal electrodes, without adversely affecting device performance or reducing the interpixel resistance further, to provide suitable contact surface for electronic read-out circuitry attachment.